TV station owner Sinclair Broadcast Group Inc. said Monday it is acquiring Tribune Media Co. for $3.9 billion, a deal that would create a behemoth with more negotiating leverage over programmers and distributors and the ability to launch new channels or wireless streaming services.

“We will be the largest broadcast group by a country mile,” Sinclair Broadcast Group Chief Executive Chris Ripley told analysts Monday. The combined companies, he said, would cover 72% of the nation’s television households and have $4.3 billion...